Interacting tunneling systems in ionic crystals

Potassium chloride crystals doped with impurity ions like Li+ are known to form atomic tunneling centers. At very low concentrations the Li+-ions can be considered as isolated and their properties can be described from a microscopic point of view. With rising concentration mutual interactions among the defects increasingly change their energy spectrum and their dynamics. We discuss various experimental results in the framework of two approximations which describe interacting tunneling systems at different defect concentrations.